1. Field of the Invention
The present invention relates to a light source device in which one or more flat substrates provided with a light emitting element on its surface side are supported by a chassis provided with a conductive flat plate surface in such a matter that a back surface of the substrate is opposed to the flat plate surface, and more particularly to a light source device which can be used as a backlight device of a transmissive liquid crystal display or an illuminating device.
2. Description of the Related Art
A transmissive liquid crystal display is provided with the backlight device as a light source on a back surface of a liquid crystal panel. Conventionally, while a CCFL (Cold Cathode Fluorescent Lamp) has predominated as the light source of the light source device used as the backlight, an LED (Light Emitting Diode) is expected to be used as the light source instead of the CCFL because a technology of the LED has been developed. For example, Japanese Unexamined Patent Application Publication No. 2003-207780 (hereinafter, referred to as Document 1) discloses a backlight device using an LED element as a light emitting element. According to Document 1, a light source device can implement surface irradiation over a whole screen by diffusing the desired number of LED elements uniformly on a substrate. In addition, the surface irradiation can be implemented by arranging a plurality LED modules of a linear light source in which the plurality of LED elements are linearly arranged on a substrate, as disclosed in Japanese Unexamined Patent Application Publication No. 2008-53062. In addition, in the case where the plurality of LED elements are arranged on the substrate, there is a method for arranging the LED elements in parallel like in Document 1, and a method for arranging a predetermined number of LED elements in series as disclosed in Japanese Examined Utility Model Application Publication No. 62-34468.
Therefore, the LED module in which the plurality of LED elements are linearly arranged on a substrate and connected in series is provided with an anode terminal 40 and a cathode terminal 41 in the vicinity of an end side of an insulation substrate 20, to draw out anodes and cathodes at both ends of the plurality of LED elements 1 connected in series as schematically shown in FIGS. 11A and 11B. According to the case shown in FIGS. 11A and 11B, the anode terminal 40 and the cathode terminal 41 are adjacently arranged on one end of a substrate 2. FIGS. 11A and 11B show an LED module 50 in which eight LED elements are connected in series, as one example. Here, FIG. 11A shows a state before the LED elements 1 are mounted, and FIG. 11B shows a state after the LED elements 1 have been mounted.
In the LED module 50, nine connecting wires 31 to 39 to connect the eight LED elements 1 in series are formed on the insulation substrate 20, and the connecting wires 31 to 39 are apart from each other and their ends are adjacently and sequentially arranged. One end of the first connecting wire 31 is connected to the anode terminal 40, and the other end is connected to an electrode 42 connected to an anode of the LED element 1. Each one end of the second to eighth connecting wires 32 to 38 is connected to the electrode 43 connected to the cathode of the LED element 1, and each other end thereof is connected to an electrode 42 connected to an anode of the LED element 1. One end of the ninth connecting wire 39 is connected to the cathode terminal 41 and the other end is connected to the electrode 43 connected to the cathode of the LED element 1. Thus, by connecting the anode and cathode of the LED element to the electrodes 42 and 43 provided at the adjacent ends of the adjacent connecting wires, respectively, the LED module is configured as the module in which the eight LED elements 1 are forwardly connected in series between the anode terminal 40 and the cathode terminal 41.
FIGS. 12 and 13 are configuration views of an LED light source device in which the LED modules 50 shown in FIGS. 11A and 11B are arranged by 2×4 in the form of a matrix and the LEDs are arranged on a plane surface. For example, as shown in FIG. 12, according to a configuration in which the LED modules 50 are arranged in parallel, a control substrate 51 to drive the LED elements of each LED module 50 and to control its emitted light is provided outside, and the anode terminal 40 and the cathode terminal 41 of each LED module 50 are connected to the control substrate 51 through an external wire 52. Furthermore, as shown in FIG. 13, according to a configuration in which four groups each composed of the two LED modules 50 connected in series are arranged in parallel, the one anode terminal 40 is connected to the other cathode terminal 41 between the two adjacent LED modules 50, and the other anode terminal 40 and the other cathode terminal 41 are connected to the control substrate 51 through the external wire 52. In either configuration, the light emission of each LED module 50 can be controlled by the control substrate 51. In addition, regarding the configuration shown in FIG. 13, since the LED modules 50 are connected in series, the number of terminals required for controlling the control substrate 51 can be reduced.
Recently, the televisions are required to be reduced in thickness and weight. The reduction in weight can be implemented by reducing the number of components and reducing the size of the component in general. As a result, the substrate of the LED backlight is also reduced in area and in weight by arranging the strip-shaped substrates in the form of the array as shown in FIG. 12 instead of spreading the LED elements all over the surface in the form of an array, whereby the weight reduction is implemented. Furthermore, the substrate is reduced in weight by reducing a thickness.
In addition, a simple method is employed in fixing the LED substrate with a view to reducing labor cost and because the LED substrate is reduced in size and weight. For example, instead of fixing the LED substrate with screws over the whole surface, a simple method such that both ends of the strip-shaped substrate are fixed with rivets (allowance is provided as compared with the fixing with screws) is used to implement inexpensive production.
Furthermore, as for the reduction in thickness, it is required to reduce a distance from the chassis of the backlight device to a liquid crystal panel surface in the market. As a result, a gap between the chassis of the backlight device and the LED substrate is considerably narrowed.
Because of the above reduction in weight, the LED substrate could be displaced even with small force. As a matter of course, since the LED substrate is fixed to the chassis, it is not largely moved, but the LED substrate could be slightly moved with small force because the substrate itself is bent due to reduction in thickness to reduce its weight, and there is an allowance gap of the small rivet.
The inventor of this application has confirmed that the LED backlight device generates a noise (sound) under the above circumstances. Some level of sound reaches as high as 30 dB although depending on the size of the backlight device (corresponding liquid panel screen size), which negatively affects the viewing and listening of the television serving as audio visual (AV) equipment. Further examination has found that this sound phenomenon is generated due to contact between the LED substrate and the chassis provided on its back surface.
By the way, when the LED elements are connected in series on the LED substrate, the same current flows in the LED elements, so that an essentially great benefit is that brightness of the LED element is uniform. Meanwhile, when the LED elements are connected in parallel, the problem is that the brightness of the LED element is not uniform unless the LED elements are driven separately. In addition, when the LED elements are driven separately, a wire for driving each LED element is needed, and the number of terminals of the drive circuit is increased, which is a problem for the LED substrate having the many LED elements. Therefore, the above problem is solved by connecting the LED elements in series.
However, when the LED elements are connected in series, a drive voltage of the LED elements becomes high, so that a potential fluctuation of the LED substrate due to the on/off of the LED element also becomes large, which has been turned out to cause the sound to become conspicuous.